Jan Egeland, the head of the Norwegian Refugee Council, and UN officials said they were willing to talk with any faction, including the Islamic State group (IS), if it helped deliver life-saving aid to victims of the conflict.

Peace talks, delayed by disputes over the make-up of the opposition delegation, are due to open on Friday.

United Nations envoy Staffan de Mistura, who is brokering the negotiations, has said that securing space for humanitarian workers was a top priority, with an estimated 4.5 million people living in besieged or restricted areas without regular access to aid.

Egeland, a former UN under-secretary general, said government and rebel forces could immediately pave the way for massive aid deliveries if they stopped playing "games."

He urged the rival parties to "tear down the Berlin Wall of hindrances they have built between us who can help and millions of defenceless civilians in the conflict zones."

With the wrangling over the opposition delegation persisting, the UN noted that millions of Syrians who need emergency aid are in areas controlled by IS, which is not part of any internationally-recognised peace process.

As any deal on humanitarian convoys would not apply to the extremist group's territory, Egeland said he was "willing to speak to anyone to secure the access of me and my colleagues to people in need."

John Ging, operations chief at the UN's humanitarian agency (OCHA), agreed, telling reporters in response to a question about IS that the UN would "engage with any party, anywhere, anytime for the purpose of gaining access", to those who need aid.

"There are certain parties, certain actors, who will not engage with us," he said, in an apparent reference to IS.

The peace talks, where opposition and government representatives will initially be seated in separate rooms, are expected to last six months, with the first round slated to take two to three weeks.

De Mistura's office said invitations to the approved negotiators were sent out on Tuesday, without providing any details about the delegations.